I always end up with the best results when I have a car that is easy to drive. A car that is under steering or oversteering is not easy to drive. If the car is easy to drive, I feel like I can hit the same line repeatedly, which might be slightly slower on the fast lap but will allow me to push the car more and make better time over the course of the run. One error can remove any advantage that a faster but more edgy car has over one which is relatively neutral...
I always find this to be the case. I have way too many cars, and try to drive too many of them on any given night rather than just sticking with one and really focusing on that. The car I hit my fastest lap with is never the car I can perform the best in a race with. One collision or wall hit will negate a bunch of fast laps.